Strong ferromagnetism of g-C3N4 achieved by atomic manipulation

Two-dimensional (2D) metal-free ferromagnetic materials are ideal candidates to fabricate next-generation memory and logic devices, but optimization of their ferromagnetism at atomic-scale remains challenging. Theoretically, optimization of ferromagnetism could be achieved by inducing long-range magnetic sequence, which requires short-range exchange interactions. In this work, we propose a strategy to enhance the ferromagnetism of 2D graphite carbon nitride (g-C3N4), which is facilitating the short-range exchange interaction by introducing in-planar boron bridges. As expected, the ferromagnetism of g-C3N4 was significantly enhanced after the introduction of boron bridges, consistent with theoretical calculations. Overall, boosting ferromagnetism of 2D materials by introducing bridging groups is emphasized, which could be applied to manipulate the magnetism of other materials.

1. Authors reported that "Therefore, sp2 hybridized B was chosen to be introduced into the g-C3N4 motif to facilitate the magnetic moment exchange interactions (Figure 1a-b).". The strategy is interesting. However, it is not clear for readers to understand the mechanism in Figure 1b. The authors could improve the schematic figure.
2. There is no conclusive experimental evidence that boron is introduced at the bridge site as sp2 hybridization. Thus, the X-ray absorption near-edge structure spectroscopy is suggested to be applied to characterize the coordination environment of boron. 3. In Figure 4d, the magnetization curve has not reduced to zero yet, so the Curie temperature would be higher than 350 K. Authors could measure the M-T curve at higher temperature to obtain the Curie temperature. 4. Inductively coupled plasma is suggested to be used to eliminate the influence of magnetic impurity elements. 5. Authors mentioned that "Over the synthetic process, two -B(OH)n-incorporation scenarios could be proposed, one is the in-planar bridging -B(OH)-groups (Figure 1c), and the second is the terminal bonding of -B-(OH)2 groups." Is there any experimental evidence to support this point? Which one is energetically favorable? 6. In Figure 5b, there is an obvious reduction of the spin polarization density discovered around N defects compared with that in Figure 5a. I suggest the authors to give the explanation on it. 7. In Figure S7, authors provided the comparation of magnetic properties between bulk C3N4 and g-C3N4 with N defects. How does the magnetic property depend on the configuration of the g-C3N4 with in-planar bridging -B(OH)-groups?
Reviewer #3: Remarks to the Author: The authors report experimental and theoretical data on magnetic properties of graphite carbon nitride layers related to introduced boron. Layers are produced via a supercritical CO2 treatment where boron is added. A wide range of experimental techniques, including TEM, XPS, FTIR and magnetometry are used for the structural and magnetic characterization. Furthermore, the study includes DFT simulations focusing on boron configurations that yield long-range ferromagnetic coupling in the layers. The outcome of the study is interesting. Inducing magnetism in carbon-nitride layers is an appealing approach und potentially gives access to magnetic materials based on abundant elements. Overall, however, publication of the results seems to be premature and the high levels of quality set by Nature Communications are, in my opinion, not met. I am mainly missing a more qunatitative evaluation of the results, especially since high-temperature ferromagnetism in Bdoped g-C3N4 has been reported before (Ref. 18).
-I am mainly lacking a detailed discussion comparing magnetic moments per unit cell from DFT and the experimentally determined magnetization considering the amount of B detected by, e.g., XPS. This would be the basis for discussing in more detail the fraction of B inducing a magnetic moment and the overall density of B in the films.
-The ferromagnetic response without B is surprisingly high (2 memu/g) and seems to be qualitatively different from other reports (ca. 0.4 memu/g in Ref. 18). How can this be understood?

To Reviewer 2:
Reviewer #2 (Remarks to the Author): In the manuscript, entitled "Strong ferromagnetism of g-C3N4 achieved by atomic manipulation", Du et al, proposed a scheme for enhancing the ferromagnetism of graphite carbon nitride (g-C3N4), which is facilitating the short-range exchange interaction by introducing in-planar boron bridges. The topic and idea dealt here seems to be fascinating for the wide audience in the 2D material science community. However, regrettably, the manuscript is not suitable for publication in its current form, owing to the results are not strong enough to support what the authors want to insist in. I have provided some questions and comments for the authors below.
1. Authors reported that "Therefore, sp 2 hybridized B was chosen to be introduced into the g-C3N4 motif to facilitate the magnetic moment exchange interactions (Figure 1a-b)". The strategy is interesting. However, it is not clear for readers to understand the mechanism in Figure 1b. The authors could improve the schematic figure.

Reply：We agree that the mechanism of Figure 1 might not be clear to all audiences. Therefore, a detailed mechanism for the formation of 2D g-C3N4 nanosheets (B-C3N4-X MPa) with in-planar bridging -B(OH)-was added in the
Scheme S1 of the manuscript (Fig. Reply 1)

Fig. Reply 1. The proposed mechanism of borate decorated 2D g-C3N4 nanosheets with inplanar bridging -B(OH)-group.
2. There is no conclusive experimental evidence that boron is introduced at the bridge site as sp 2 hybridization. Thus, the X-ray absorption near-edge structure spectroscopy is suggested to be applied to characterize the coordination environment of boron.

Fig. Reply 2. B K-edge XANES spectra of B-C3N4-16 MPa.
3. In Figure 4d, the magnetization curve has not reduced to zero yet, so the Curie temperature would be higher than 350 K. Authors could measure the M-T curve at higher temperature to obtain the Curie temperature.

Reply: We apologize that due to the instrumental limitation, the FC-ZFC characterization is not totally completed in the last version. Therefore, the FC-ZFC magnetization curves of B-C3N4-16
MPa were characterized with a different SQUID magnetometer (Fig. Reply 3).

Fig. Reply 3. FC-ZFC magnetization curves of B-C3N4-16 MPa in external magnetic field of 100 Oe. Inset: ΔM = MFC − MZFC curve.
4. Inductively coupled plasma is suggested to be used to eliminate the influence of magnetic impurity elements. (Table S3 and Table Reply 1 5. Authors mentioned that "Over the synthetic process, two -B(OH)n-incorporation scenarios could be proposed, one is the in-planar bridging -B(OH)-groups (Figure 1c), and the second is the terminal bonding of -B-(OH)2 groups." Is there any experimental evidence to support this point? Which one is energetically favorable?

Reply: We agree that the two scenarios need to be supported with more experimental results, which are critical for this work. To verify the hypothesis, solid-state 11 B MAS NMR was carried out to characterize the bridging -B(OH)-groups and the terminal -B-(OH)2 groups. In the 11 B
NMR spectrum (Figure 3f and Fig. Reply 4) Theoretical calculations were conducted to tell which B incorporation (bridging, terminal) is more favorable (Figure S1 and Fig. Reply 5), which suggest the formation of bridging -B(OH)is more favorable.

Fig. Reply 5. The DFT calculated free energy diagram for the formation of -B(OH)nincorporation.
6. In Figure 5b, there is an obvious reduction of the spin polarization density discovered around N defects compared with that in Figure 5a. I suggest the authors to give the explanation on it. Figure 5b. The explanation is discussed in the manuscript as well.

Reply: Since the in-planar bridging -B(OH)-groups connect two tri-s-triazine units, the spin polarization density around N defects is free to transfer between tri-s-triazine units through -B(OH)-bridges by electron and magnetic moment delocalization. Such delocalization process could reduce the spin polarization in
7. In Figure S7, authors provided the comparation of magnetic properties between bulk C3N4 and g-C3N4 with N defects. How does the magnetic property depend on the configuration of the g-C3N4 with in-planar bridging -B(OH)-groups?

Reply: The magnetism is originated from the unpaired electron from the N defects, which is required for the magnetism and enhanced by the -B(OH)-bridges in this work. Specifically, over the SC CO2 treatment and boron incorporation discussed in the manuscript, the N defects is expected to be generated, leading to the ferromagnetism as observed. The ferromagnetism is significantly enhanced by the -B(OH)-bridges by facilitating exchange interaction. However, introducing in-planar bridging -B(OH)-on g-C3N4 alone (without N defects), is expected to be diamagnetic: lacking of unpaired electron from N defects, no ferromagnetism is anticipated.
Reviewer #3 (Remarks to the Author): The authors report experimental and theoretical data on magnetic properties of graphite carbon nitride layers related to introduced boron. Layers are produced via a supercritical CO2 treatment where boron is added. A wide range of experimental techniques, including TEM, XPS, FTIR and magnetometry are used for the structural and magnetic characterization. Furthermore, the study includes DFT simulations focusing on boron configurations that yield long-range ferromagnetic coupling in the layers.
The outcome of the study is interesting. Inducing magnetism in carbon-nitride layers is an appealing approach und potentially gives access to magnetic materials based on abundant elements. Overall, however, publication of the results seems to be premature and the high levels of quality set by Nature Communications are, in my opinion, not met. I am mainly missing a more quantitative evaluation of the results, especially since high-temperature ferromagnetism in Bdoped g-C3N4 has been reported before (Ref. 18).
-I am mainly lacking a detailed discussion comparing magnetic moments per unit cell from DFT and the experimentally determined magnetization considering the amount of B detected by, e.g., XPS. This would be the basis for discussing in more detail the fraction of B inducing a magnetic moment and the overall density of B in the films.

Reply: The ferromagnetic characterizations and XPS results demonstrate that the experimentally determined magnetization positively correlate with the amount of B (Figure 4a and Table S1), where B-C3N4-16 MPa with highest amount of B (2.84 wt%) exhibit strongest ferromagnetism with Ms of 0.043 emu g -1 . Such experimental trend is consistent with theoretical calculation, where bridging -B(OH)-is expected to enhance the ferromagnetism through short-range exchange interactions. The contents have been added in the revised manuscript.
-The ferromagnetic response without B is surprisingly high (2 memu/g) and seems to be qualitatively different from other reports (ca. 0.4 memu/g in Ref. 18). How can this be understood?

Reply: Thanks for the suggestions. The references have been mentioned and discussed in the revised version.
Reviewers' Comments: Reviewer #2: Remarks to the Author: Although the authors have added additional experimental results and first-principles calculations, the mechanisms of the ferromagnetism in B-C3N4-X MPa originating from N defects and the longrange magnetic sequence forming through the introduction of B bridges are not well illustrated. The borate-functionalized 2D amorphous g-C3N4 nanosheets (B-C3N4-X MPa) were fabricated by chemical synthesis, magnetic impurities would easily exist in the sample. For example, oxygen containing functional groups would be easily introduced in g-C3N4 nanosheets, which will also possibly lead to ferromagnetism. Importantly, the saturation magnetization (MS) of B-C3N4-16 MPa at 300 K (0.043 emu g-1) is still too weak, although it is higher than that in B-doped g-C3N4 nanosheets [Sci. Rep. 2016, 6, 35768]. For 2D metal-free ferromagnetic materials, many works have reported much higher MS, e.g., 0.39 emu/g in graphene oxide nanoribbons, 0.4 emu/g in nitrogen doped graphene and 0.71 emu/g in carbon nitride sheets. This paper is not recommended because neither it contains enough data to establish their results nor be able to provide a significant breakthrough in 2D metal-free ferromagnetism. I think this work is not suitable for Nature Communications.
Reviewer #3: Remarks to the Author: Authors incorporoted additional experimental results and discussion in the new version of the manuscript. Points raised by reviewers were considered in an convincing way and the manuscript and support of the conclusions is now significantly improved. On this basis I can suggest publication in Nature Communications.

Many thanks to the reviewers for their valuable comments and suggestions. The followings are the point-by-point answers to the concerns:
To Reviewer 2: Reviewer #2 (Remarks to the Author): Although the authors have added additional experimental results and first-principles calculations, the mechanisms of the ferromagnetism in B-C3N4-X MPa originating from N defects and the longrange magnetic sequence forming through the introduction of B bridges are not well illustrated. The borate-functionalized 2D amorphous g-C3N4 nanosheets (B-C3N4-X MPa) were fabricated by chemical synthesis, magnetic impurities would easily exist in the sample. For example, oxygen containing functional groups would be easily introduced in g-C3N4 nanosheets, which will also possibly lead to ferromagnetism. Importantly, the saturation magnetization (MS) of B-C3N4-16 MPa at 300 K (0.043 emu g -1 ) is still too weak, although it is higher than that in B-doped g-C3N4 nanosheets [Sci. Rep. 2016, 6, 35768]. For 2D metal-free ferromagnetic materials, many works have reported much higher MS, e.g., 0.39 emu/g in graphene oxide nanoribbons, 0.4 emu/g in nitrogen doped graphene and 0.71 emu/g in carbon nitride sheets. This paper is not recommended because neither it contains enough data to establish their results nor be able to provide a significant breakthrough in 2D metal-free ferromagnetism. I think this work is not suitable for Nature Communications. (0.043 vs. 0.026, 65.3% higher) Figure Reply S1-S2).

In conclusion, the results suggest that the ferromagnetic enhancement in B-C3N4-X MPa is mainly originated from the -B(OH)-bridges introduced to the sample instead of the O containing groups. Importantly, as discussed in the manuscript, the -B(OH)-groups enhance the magnetism by establishing long-range magnetic sequence in the sample, which is a quite unique mechanism comparing to the magnetism from oxygen containing groups.
The aforementioned discussion and results have also been added to our updated Supplementary Information.